Matt, you've gotten lots of good inputs, but I see it a little differently than some of them. Based on the information Yogiboy supplied about the effective mass of the tonearm (8.5 grams) and the compliance of the cartridge (15, although I haven't been able to find an indication of what the units are for that number, which can make a significant difference), and also taking into account the weight of the cartridge (8.5 grams), it seems to me that if anything the resonant frequency of the combo is likely to be above, not below, the 8 to 12 Hz area or thereabouts that is generally considered to be ideal.
That will tend to reduce, not increase, sensitivity to most warp frequencies, and also to modulations caused by off-center records. What it will increase is sensitivity to very deep bass content that may be on the record. So my question is does the pumping issue manifest itself on most or all recordings and passages, or just on those that seem to have significant content in the bottom octave?
Regarding my other point about the connection of the sub's ground wire, ideally it should be connected to a circuit ground point on the amp, which in this case isn't provided. None of the 12 binding posts on the rear of the amp are at circuit ground potential, as can be determined by close examination of the wiring diagrams for the autoformers, as shown on page 14 of the manual for the MC452 (linked to on
this page), which presumably is conceptually similar in this respect to your MC352.
In some cases the result of connecting the ground wire of a powered sub to a negative output terminal of an amp having balanced outputs can be severe hum, activation of protection mechanisms, or even damage to the amp as a result of having an output terminal bearing a signal shorted to ground. In many cases, though, fortunately including this one, the designs are such that none of those things occur.
However, looking at the autoformer wiring diagrams I referred to, I suspect (given that you appear to be using a single sub for both channels), that you are not providing the sub with a sonically optimal pair of signals. Specifically, it appears that half of the signal (either the positive half or the negative half, at any given instant) that is received by the sub from the channel to which the ground wire is not attached will actually be derived from the other channel. Which doesn't sound like a recipe for ideal sonics.
Although this is obviously not related to the pumping problem, since the woofers on the main speakers pump even when the sub is disconnected, at some point you might consider connecting the ground wire to a chassis screw on the amp. Depending on the internal grounding configuration of the amp and the sub, that may work fine (it does in many cases), or it may result in a lot of hum. (Doing that, btw, would necessitate increasing the sub's level control by about 6 db). Or you might even consider connecting the sub at line level, to the preamp, although to do that with your particular sub you would need the 12 db attenuator REL supplies (or at least used to supply) for use when a single sub of that model is provided with two input channels at line level, since its two line level inputs have gains differing by 12 db.
Regards,
-- Al
